The present invention relates generally to cold deforming of metal and more particularly to a method for radially forging a tubular workpiece having a circular cross section.
Conventional radial forging methods employ swaging. In swaging, a plurality of dies (e.g., 2-4 dies) rotate in steps around a tubular workpiece, delivering hammer blows to the workpiece to deform the latter to a desired outside diameter at an axial location on the workpiece aligned with the swaging dies. Alternatively, the workpiece can be rotated, in steps, while the dies remain in place, hammering the workpiece as it is rotated. Typically, the rotation is in increments of two to three degrees, until the entire workpiece, at a given axial location thereon, has been radially forged to the desired outside diameter. The workpiece is then advanced in an axial direction until another axial location on the workpiece is aligned with the dies which are then actuated to hammer at the new location on the workpiece to deform the latter to the desired outside diameter.
In swaging, the outside diameter of the workpiece is determined when the dies are all circumferentially touching at their radially inward ends at the end of a swaging operation. Thus, for a given set of swaging dies, only one outside diameter can be formed with any accuracy, and this corresponds to the diameter of the circle defined by the swaging dies when they are all circumferentially touching at their radially inward ends.
Another type of cold deforming operation is shrink forming. Shrink forming utilizes a multiplicity of circumferentially arranged dies (e.g., 12 dies) each having a curved or arcuate die face with a pair of side edges. The dies engage a tubular workpiece around its exterior, at a given axial location on the workpiece, and squeeze the exterior of the workpiece, in a first shrink forming pass, until the outside diameter of the workpiece is reduced to the desired amount. The dies then retract, and either the set of dies or the workpiece is rotated slightly. The squeezing operation is then repeated, in a second shrink forming pass. The dies then retract, and the workpiece is advanced axially until a new axial location on the workpiece is aligned with the dies which then engage and squeeze the workpiece at the new axial location.
A mandrel may be employed on the inside of the workpiece during shrink forming, and the mandrel determines the inner diameter of the workpiece at the end of the shrink forming operation. Conventionally, in shrink forming operations of the type described above, either the wall thickness or both the wall thickness and the length of the tubular workpiece increase as the outside diameter decreases. When a mandrel was employed, the increase in wall thickness and length stopped once the mandrel was contacted by the inside surface of the tubular workpiece, thus determining both the inner and outer diameters of the workpiece and its wall thickness. The wall thickness of the workpiece was not decreased in conventional shrink forming operations, either with or without an internal mandrel. The area of the workpiece engaged by the dies on a shrink forming pass (and by the mandrel at the end of the shrink forming pass) were relatively large.
An example of a shrink forming method and apparatus is disclosed in Luedi et al. U.S. Pat. No. 3,461,710, and the disclosure thereof is incorporated herein by reference.
There is normally a gap between adjacent shrink forming dies at the beginning of the shrink forming operation, and the gap decreases as the dies move radially inwardly during the shrink forming operation. This gap is at a minimum at the end of the shrink forming operation, when the workpiece has the desired outside diameter, but it is not desirable that the gap between the shrink forming dies then be totally eliminated because this may cause damage to the dies or their attachments, and there is no great benefit to have the dies exactly touch then. When dies touch, the side edges of adjacent die faces are in contact.
The unengaged surface area of the workpiece, at the gap between the dies, is relatively small compared to the surface area of the workpiece engaged by the face of a shrink forming die. This unengaged surface area of the workpiece does not undergo shrinking uniformly with the adjoining surface areas of the workpiece which are engaged by the die faces during the first shrink forming pass. Accordingly, a shrink forming operation should employ at least two passes, with either the workpiece or the set of dies being rotated relative to the other so that, on a second pass, subsequent to the first pass, the surface areas of the workpiece at the gaps between the dies, which were unengaged during the first pass, are engaged by the shrink forming die faces.
There is, however, a problem which arises when a shrink forming operation involves at least two shrink forming passes separated by a rotating step. More particularly, the arcuate die face engaging the workpiece has a curvature corresponding to the desired final outside diameter of the workpiece, whereas, at the beginning of the shrink forming operation, the workpiece has a larger outside diameter and a corresponding larger curvature than the arcuate die face. Each curved die face has a pair of side edges each adjacent a gap between dies. These side edges of the curved die face will engage the workpiece before it is engaged by that part of the curved die face between the side edges, and this will concentrate the shrink forming pressure at the two side edges, at the beginning of the shrink forming operation. As a result, metal is extruded radially outwardly in the unengaged gap between adjacent dies, during the first pass. When the dies are rotated between passes, they arrive at a position at which they can engage the previously unengaged surface areas of the workpiece, i.e. the areas containing the extruded metal. As a result, on the second pass, the metal extruded on the first pass is folded or lapped over, and this produces an imperfection in the surface of the workpiece.
Another problem which can arise when a shrink forming operation involves a pair of passes separated by a rotating step, as described above, is that the workpiece can undergo a torsional or twisting type of deformation, rather than undergoing a strictly radial type of deformation. Although torsional deformation may not change the shape of the workpiece, it is an unproductive type of deformation. It consumes work and energy and generates heat without accomplishing anything. Moreover, although straight radial deformation may require an annealing operation after a number of shrink forming passes to make the workpiece deformable again, torsional deformation requires more frequent annealing.
It would be desirable to be able to use a shrink forming operation to produce articles such as jet engine shafts having outer and inner diameters and wall thicknesses which vary in an axial direction along the shaft. Previously, such articles have been produced by extensive machining of a solid bar or a tube, inside and out, but, in such products, a middle portion thereof sometimes has a larger inner diameter than end portions thereof, and this creates difficulties in the machining operation. In addition, extensive machining is expensive and produces relatively large amounts of scrap material which is wasteful.
With swaging, an inner mandrel may be employed to obtain variations in inside diameter and to provide variations in wall thickness, but only one controlled outside diameter can be obtained with a given set of swaging dies, and if swaging is terminated before the dies circumferentially touch at their radially inward ends, there can be no control on the outside diameter with any precision.